Refining of reclaimed rubber



INVENTOR. CHARLES H. CAMPBELL 1 fo r ,1 e

C H CAMPBELL REFINING OF RECLAIMED RUBBER Filed May 18, 1949 Oct. 3, 1950 Patented Oct. 3, 1950 UNITED STATES PATENT OFFICE 2,524,375 REFINING F RECLAIMED RUBBER Charles Herbert Campbell, Kent, Ohio Application May 18, 1949, Serial No. 94,036

(01. lit-4s) 2 Claims. a 1 This invention relates to the refining of reclaimed rubber.

After reclaimed rubber has been digested, i. e.,

after it has been heated with a softening and swelling oil or oil-resin blend, with or without caustic, chemical softeners, etc., it is sheeted out on a refining mill. A refining mill has two rolls. These rolls are both hollow and are cooled with available cooling water. Sometimes cool well water is used.

When the reclaim is worked on the refining mill the diiference in the circumferential speed of the two rolls effects a grinding action, and this generates heat rapidly, particularly with reclaims containing GR-S which customarily contains more carbon black, than natural rubber. It is the purpose of the cooling water to prevent the rolls and the rubber from getting too hot. During the milling, the reclaim is sheeted out and the tailings separated. If the rubber in the bank of the mill is permitted to become too hot it becomes doughy and the tailings are not properly separated because the sheeted rubber is so soft it permits undevulcanized particles to pass with it through the bite of the rolls. Also, if the rubher in the bank becomes too hot, the tensile strength of the reclaim is reduced and difliculty is encountered with the sheet of reclaim breaking as it is transferred from the mill to the windup roll.

The scrap available for reclaiming now runs from 60 to 70 per cent synthetic and the prospect is that the percentage of GR-S in the available scrap will increase, rather than decrease. The per cent of cold-process GR-S will increase very little until the plants are equipped for it, and it will probably be two years before this scrap runs in any quantity to the reclaiming plants. The reclaim may eventually contain as much as 90 per cent or more of GR-S.

The reclaiming of all synthetics is improved by drastically reducing the temperature of the rolls of the refining mill. This not only conserves the value of the synthetic portion of the scrap, but also prevents the natural-rubber portion from becoming doughy and sticky. When the refining mills are operated at the higher temperatures now employed, especially the higher temperatures used in the hot summer weather, the natural-rubber content of the scrap becomes mushy and doughy and allows any tailings from the synthetic to run through. This is corrected by cooling the rolls of the refining mill with brine or other refrigerant which has a temperature of 40 to 50 F., and keeping the cylindrical surface of the N0. 1

roll at a temperature which is preferably to F., depending upon the time of year, and keeping the cylindrical surface of No. 2 roll within 15 to'20 F. of this temperature and not over F. These temperatures cannot be maintained in hot weather except by refrigerant. Such low-temperature milling conserves the natural factors inherent in both the synthetic and natural-rubber portions of the scrap, each of which may comprise from fifteen or twenty per cent up to eighty or eightv-five per cent of the entire scrap. A reclaim of good tensile is produced, so that the difliculty now experienced with breakage or hot cracking is largely eliminated, and the sheet produced is smooth and free from holes.

The operation is so elficientthat instead of limiting the bite of the mill rolls to the usual thickness of only about .003 inch, it is possible to increase it to .0035 inch or .004 inch under best working conditions. This increase results in an increase in the volume of the rubber treated on a single mill and operating costs are thus reduced.

In the past it has been common practice, particularly in very hot weather, and especially in reclaiming GR-S, to remove a bank of hot, doughy reclaim from the mill for a long enough time to allow it to cool and then return it to the mill to complete the refining of it. By operating at the lower temperatures herein contemplated such interruption of the refining operation is no longer necessary and GR-S can be refined as a continuous operation.

The amount of refrigerant used iscontrolled to meet the requirements of the type of scrap being treated, etc. The refrigerant may be water, or a brine such as calcium chloride solution, etc. "It will advantageously contain a small amount of alkali to prevent or limit corrosion of the inner surface of the rolls. The refrigerant may be cooled by any suitable refrigerating means such as that which embodies an ammonia compressor or the like. Any means for circulating the refrigerant through the mill will be satisfactory. The accompanying drawings are more or less schematic but suggest equipment which may be used.

In the drawings:

Fig. 1 is a side View of a refining mill partly broken away; and

Fig. 2 is a longitudinal section through the rear roll of the mill.

The No. l and No. 2 rolls of the mill are identified herein by the reference numerals and 2 respectively. They are hollow and contain the refrigerant 3 which is slowly circulated through them, re-cooled, and then re-circulated. The fiow of the refrigerant through the separate rolls is so controlled that the cylindrical surface of the No. 1 roll is maintained at preferably about 105 F. and the cylindrical surface of the No. 2 roll is maintained at preferably about 120 although some stocks may require a temperature 10 F. higher. The over-all temperature range of the two rolls is therefore about 105 to 130 F. The feed line to each roll is equipped with a control valve 4 which may be operated manually or may be equipped with thermostatic means for maintaining the roll surface within prescribed temperature limits.

The No. 2 roll is geared to revolve faster than the No. 1 roll. The reclaim from the bank 5' is carried around the No. 2 roll as a sheet 6 which is removed by the knife 1 and transferred to the wind up 8.

The drawing illustrates simple circulation means. The feed pipe 10 connects with the stationary enlargement l I which is held in the bearing which supports the roll. The brine enters the roll through the perforated pipe l2 which leads toward the bottom of the roll. Suitable packing I3 surrounds the hollow trunnion M. At the other end of the roll is an over-flow l5 for the used brine which is returned to the refrigerating unit through the line 96. Both rolls may be similarly equipped.

There is nothing novel about the construction of the mill or the piping connections. The novelty lies in the process.

The following example illustrates the inven tion.

Ground scrap composed of 70 parts of cured GR-S and 30 parts of cured natural rubber are treated in a digester with a dilute solution of zinc chloride and sufficient water to cover the scrap (usually 2 to 2 /2 pounds of water per pound of scrap). There is also added to the digester '17 to per cent (based on the weight of the scrap) of an oil-resin blend (which will vary with per cent of GR-S in the scrap, etc.) This is formed of (a) 50 parts by weight of dipolymer oil, (b) parts by weight of a resin of 500 to 800 molecular weight composed essentially of resinous polymer of olefins and diolefins, and (c) 25 parts by weight of a resin under 500 molecular weight composed of coumarone polymers, coumarone-indene polymers and indene polymers. This mixture is digested and then blown down in the usual manner. The resulting reclaim is then worked on the refining mill. With an atmospheric temperature of 70 F. the No. 1 roll.

of the refining mill is cooled to a surface temperature of 110 F. and the No. 2 roll is cooled to a surface temperature of 15 to 20 degrees higher, and these temperatures are maintained, with some variation, during the refining operation. The flow of refrigerant to each roll is controlled by the operator. Calcium chloride brine containing a little alkali and cooled to 40 F. is circulated through the rolls of the mill to maintain these temperatures.

What I claim is:

1. In the process of reclaiming a mixture composed essentially of natural rubber and GR-S synthetic rubber and containing at least 15 per cent of each, which includes refining the reclaimed mixture between two rolls of a refining mill with the No. 1 roll revolving at a slower speed than the No. 2 roll to produce a grinding action on the reclaim as it is sheeted out between the two rolls for transfer to the wind-up 'roll, the improvement which consists of internally cooling the No. 1 roll to maintain the temperature of its cylindrical surface at substantially to F. and internally cooling the No. 2

roll to maintain its cylindrical surface at a temperature of substantially to F., thereby preventing the natural rubber portion of the mixture from becoming doughy and sticky and preventing the tensile strength of the reclaim from being reduced to a point where difficulty is encountered with the sheet of reclaim breaking as it is transferred from the mill to the windup roll.

2. In the process of reclaiming a mixture composed essentially of natural rubber and GR-S synthetic rubber and containing at least 15 per cent of each, which includes refining the reclaimed mixture between two rolls of a refining mill with the No. 1 roll revolving at a slower speed than the No. 2 roll to produce a grinding action on the reclaim as it is sheeted out between the two rolls for transfer to the wind-up roll, the improvement which consists of internally cooling both rolls to maintain their cylindrical surfaces between 105 and 130 F., thereby preventing the natural rubber portion of the mixture from becoming doughy and sticky and preventing the tensile strength of the re-- claim from being reduced to a point where difficulty is encountered with the sheet of reclaim breaking as it is transferred from the mill to the wind-up roll. I CHARLES HERBERT CAMPBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 727,229 Turner May 5, 1903 866,758 Wheeler Sept. 24, 1907 1,751,116 Welton Mar. 18, 1930 

1. IN THE PROCESS OF RECLAIMING A MIXTURE COMPOSED ESSENTIALLY OF NATURAL RUBBER, AND GR-S SYNTHETIC RUBBER AND CONTAINING AT LEAST 15 PERCENT OF EACH, WHICH INCLUDES REFINING THE RECLAIMED MIXTURE BETWEEN TWO ROLLS OF A REFINING MILL WITH THE NO. 1 ROLL REVOLVING AT A SLOWER SPEED THAN THE NO. 2 ROLL TO PRODUCE A GRINDING ACTION ON THE RECLAIM AS IT IS SHEETED OUT BETWEEN THE TWO ROLLS FOR TRANSFER TO THE WIND-UP ROLL, THE IMPROVEMENT WHICH CONSISTS OF INTERNALLY COOLING THE NO. 1 ROLL TO MAINTAIN THE TEMPERATURE OF THE CYCLINDRICAL SURFACE AT SUBSTANTIALLY 105 TO 110*F. AND INTERNALLY COOLING THE NO. 2 ROLL TO MAINTAIN ITS CYLINDRICAL SURFACE AT A TEMPERATURE OF SUBSTANTIALLY TO 120 TO 130*F., THEREBY PREVENTING THE NATURAL RUBBER PORTION OF THE MIXTURE FROM BECOMING DOUGHBY AND STICKY AND PREVENTING THE TENSILE STRENGTH OF THE RECLAIM FROM BEING REDUCED TO A POINT WHERE DIFFICULTY IS ENCOUNTERED WITH THE SHEET OF RECLAIM BREAKING AS IT IS TRANSFERRED FROM THE MILL TO THE WINDUP ROLL. 